The invention relates to a water jet unit for surface working, e.g., for working a concrete surface, comprising:
a lance with a mouthpiece arranged for being coupled to a line for high-pressure water,
a holder for the lance,
an arrangement for guiding the holder in a working path,
an eccentric mechanism for the oscillation of the lance in a plane transverse to the working path in order to add width to the working. The invention also relates to a method of working a surface, e.g., a concrete surface by guiding a lance with a jet mouthpiece coupled to the high-pressure water in a path over the surface and at the same time oscillating the lance around an axis which is substantially parallel to the surface and substantially at a right angle to the path and located at a distance from the surface.
Machines and methods of this type are known from, for example, U.S. Pat. No. 6,877,930 B2 and WO 02/081823. A working sweep is made in one direction and the lance carrying the water mouthpiece is then turned to make an attack angle in the other direction and a return working sweep is made. During the working sweep the lance oscillates in a plane transverse to the directions of sweep in order to add a greater working width. Between every or after a few working sweeps the machine is advanced so that a cohesive working with a similar depth is obtained. Such machines are used, for example, for the working of the surface layer of concrete on a bridge deck before casting of a new surface layer. In this manner the uppermost reinforcement layer is exposed so that it can then be cast in fresh concrete during the new casting of the surface layer.